This paper presents a novel study of the Nature-based Solutions (NbSs) approach to analyze and propose mitigation measures for extreme floods. The study area is the Llobregat River in Catalonia, which crosses urban areas. We have selected one section in the final stretch of 4.5 km in the Barcelona Metropolitan Area. The section has suffered several damages in the last floods (e.g., 2016, 2018 and 2019), and we propose measures to reduce flood risk. Therefore, we proposed the following three specific objectives: (a) the identification of critical areas in the river stretches; (b) the identification of NbS opportunities and utilities; and (c) the mitigation measures in concrete areas from NbSs. The effectiveness of a NbS is based on the 2D simulation of the Gloria flood event (20-21 January 2020) with HEC-RAS software (version 6.0) for the better management of stormwater, and it is influenced by design and placement aspects; however, the better use of NbSs can improve flood mitigation and enhance urban resilience.